In the intervening years the consensus that human carbon emissions are altering Earth’s climate has only solidified. But in the case of the “ionospheric greenhouse effect,” the jury is still out, with decades of mixed results that include some observational evidence.

But now a new study by Mikhailov et al. makes a strong argument that most of these variations are not due to human activity but to solar activity. The ionosphere, they found, is just responding to the Sun’s 11-year cycle, in which its magnetic field slowly wraps itself into knots and produces periods of intense sunspots and flares.

The team used data from three monitoring stations across Europe over the last five solar cycles, from 1964 to 2010. These records relate to the ionosphere’s E region, a layer of particles ionized by the Sun and located around 90 to 150 kilometers high. (The E layer is perhaps best known for disappearing at night, which gives AM radio stations longer ranges as their signals bounce back to Earth off higher ionospheric layers.)

Previous studies had predicted that increased carbon dioxide levels to date should have lowered the height of the E layer by roughly half a kilometer since 1960. The maximum frequency of radio waves that it can reflect, known as E layer critical frequency (foE)—which indicates the density of electrons in the layer—should have changed by about a tenth of a percent in that time frame. Some records appeared to give the hypothesis some credence, showing ionospheric changes over the past several decades and in the predicted directions.

But the authors’ results were stark: The variations in foE were almost completely correlated to the changes in the sunspot numbers. They tracked closely not only in year-to-year changes but also within a single 11-year cycle as well as in the long-term trend between cycles. The number of sunspots and variations in foE both rose in the first 25 years of data, peaked in 1985, and fell sharply in later years. The findings strongly support the idea that over the 47 years of data, natural variability in the Sun’s cycles is mainly responsible for changes in the E layer’s properties—not carbon emissions.

Seeking more physical confirmation, the authors substituted the sunspot numbers with the total flux of extreme ultraviolet (EUV) radiation retrieved at each of the three European stations. Solar EUV radiation is what ionizes and creates the E layer. Their results were virtually identical: The changes in EUV radiation tracked nearly perfectly with the changes in foE.

These results should help us keep perspective, the team writes: Although we live in the atmosphere of Earth, the entire Earth lies in the atmosphere of the Sun—and the upper reaches of our own atmosphere are inextricably linked to the Sun’s activity. (Journal of Geophysical Research: Space Physics, https://doi.org/10.1002/2017JA023909, 2016)

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